Network Working Group T. Ts'o
Request for Comments: 2952 VA Linux Systems
Category: Informational September 2000
Telnet Encryption: DES 64 bit Cipher Feedback
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document specifies how to use the DES encryption algorithm in
cipher feedback mode with the telnet encryption option.
1. Command Names and Codes
Encryption Type
DES_CFB64 1
Suboption Commands
CFB64_IV 1
CFB64_IV_OK 2
CFB64_IV_BAD 3
2. Command Meanings
IAC SB ENCRYPT IS DES_CFB64 CFB64_IV IAC SE
The sender of this command generates a random 8 byte initial
vector, and sends it to the other side of the connection using the
CFB64_IV command. The initial vector is sent in clear text. Only
the side of the connection that is WILL ENCRYPT may send the
CFB64_IV command.
IAC SB ENCRYPT REPLY DES_CFB64 CFB64_IV_OK IAC SE
IAC SB ENCRYPT REPLY DES_CFB64 CFB64_IV_BAD IAC SE
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The sender of these commands either accepts or rejects the initial
vector received in a CFB64_IV command. Only the side of the
connection that is DO ENCRYPT may send the CFB64_IV_OK and
CFB64_IV_BAD commands. The CFB64_IV_OK command MUST be sent for
backwards compatibility with existing implementations; there really
isn't any reason why a sender would need to send the CFB64_IV_BAD
command except in the case of a protocol violation where the IV
sent was not of the correct length (i.e., 8 bytes).
3. Implementation Rules
Once a CFB64_IV_OK command has been received, the WILL ENCRYPT side
of the connection should do keyid negotiation using the ENC_KEYID
command. Once the keyid negotiation has successfully identified a
common keyid, then START and END commands may be sent by the side of
the connection that is WILL ENCRYPT. Data will be encrypted using
the DES 64 bit Cipher Feedback algorithm.
If encryption (decryption) is turned off and back on again, and the
same keyid is used when re-starting the encryption (decryption), the
intervening clear text must not change the state of the encryption
(decryption) machine.
If a START command is sent (received) with a different keyid, the
encryption (decryption) machine must be re-initialized immediately
following the end of the START command with the new key and the
initial vector sent (received) in the last CFB64_IV command.
If a new CFB64_IV command is sent (received), and encryption
(decryption) is enabled, the encryption (decryption) machine must be
re-initialized immediately following the end of the CFB64_IV command
with the new initial vector, and the keyid sent (received) in the
last START command.
If encryption (decryption) is not enabled when a CFB64_IV command is
sent (received), the encryption (decryption) machine must be re-
initialized after the next START command, with the keyid sent
(received) in that START command, and the initial vector sent
(received) in this CFB64_IV command.
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4. Algorithm
Given that V[i] is the initial 64 bit vector, V[n] is the nth 64 bit
vector, D[n] is the nth chunk of 64 bits of data to encrypt
(decrypt), and O[n] is the nth chunk of 64 bits of encrypted
(decrypted) data, then:
V[0] = DES(V[i], key)
O[n] = D[n] V[n]
V[n+1] = DES(O[n], key)
5. Integration with the AUTHENTICATION telnet option
As noted in the telnet ENCRYPTION option specifications, a keyid
value of zero indicates the default encryption key, as might be
derived from the telnet AUTHENTICATION option. If the default
encryption key negotiated as a result of the telnet AUTHENTICATION
option contains less than 8 bytes, then the DES_CFB64 option must not
be offered or used as a valid telnet encryption option. If the
encryption key negotiated as a result of the telnet AUTHENTICATION
option is greater than 16 bytes the first 8 bytes of the key should
be used as keyid 0 for data sent from the telnet client to the telnet
server, and the second 8 bytes of the key should be used as keyid 0
for data sent by the telnet server to the telnet client. Otherwise,
the first 8 bytes of the encryption key is used as keyid zero for the
telnet ENCRYPTION option in both directions (with the client as WILL
ENCRYPT and the server as WILL ENCRYPT).
In all cases, if the key negotiated by the telnet AUTHENTICATION
option was not a DES key, the key used by the DES_CFB64 must have its
parity corrected after it is determined using the above algorithm.
Note that the above algorithm assumes that it is safe to use a non-
DES key (or part of a non-DES key) as a DES key. This is not
necessarily true of all cipher systems, but we specify this behaviour
as the default since it is true for most authentication systems in
popular use today, and for compatibility with existing
implementations. New telnet AUTHENTICATION mechanisms may specify
alternative methods for determining the keys to be used for this
cipher suite in their specification, if the session key negotiated by
that authentication mechanism is not a DES key and and where this
algorithm may not be safely used.
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6. Security Considerations
Encryption using Cipher Feedback does not ensure data integrity; the
active attacker has a limited ability to modify text, if he can
predict the clear-text that was being transmitted. The limitations
faced by the attacker (that only 8 bytes can be modified at a time,
and the following 8-byte block of data will be corrupted, thus making
detection likely) are significant, but it is possible that an active
attacker still might be able to exploit this weakness.
The tradeoff here is that adding a message authentication code (MAC)
will significantly increase the number of bytes needed to send a
single character in the telnet protocol, which will impact
performance on slow (i.e. dialup) links.
7. Acknowledgments
This document was originally written by Dave Borman of Cray Research
with the assistance of the IETF Telnet Working Group.
Author's Address
Theodore Ts'o, Editor
VA Linux Systems
43 Pleasant St.
Medford, MA 02155
Phone: (781) 391-3464
EMail: tytso@mit.edu
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RFC 2952 DES 64 bit Cipher Feedback September 2000
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